In this fiscal year, we have focused on defining the functional role of myositis autoantigens in muscle differentiation and repair. As in other systemic autoimmune diseases, patients with myositis often have autoantibodies targeting self-proteins. We previously demonstrated that the dermatomyositis (DM) autoantibody CHD4 is up-regulated in regenerating muscle and that reducing levels of this protein in cultured muscle cells accelerates their differentiation. Recently, we showed that another DM autoantigen, TRIM33, is similarly up-regulated during muscle repair and seems to play a role during muscle differentiation in cell culture. During this year, we studied muscle differentiation and regeneration using mice that have no CHD4 or TRIM33 expressed in muscle cells. These experiments show that knockout of these genes results in delayed muscle regeneration following muscle cell injury. Using these mice (as well as muscle cells cultured from them), we are now determining the molecular mechanisms by which CHD4 and TRIM33 modulate muscle differentiation and repair. In addition to the basic science project described above, we also completed several clinical projects involving myositis patients who are part of the Johns Hopkins Myositis Center Longitudinal Cohort. These projects included: (a) Analyzing a longitudinal cohort of patients with anti-HMG-CoA reductase (HMGCR) antibodies. This published study showed that anti-HMGCR myositis is a chronic disease requiring long-term immunosuppression and that the disease is more difficult to control in younger patients. (b) Analyzing the relationship between muscle biopsy abnormalities and individual dermatomyositis autoantibodies. This published study showed that patients with different dermatomyositis autoantibodies have different histopathologic features. This reinforces our hypothesis that dermatomyositis is a heterogenous collection of diseases rather than a single disease. (c) Analyzing thigh muscle MRI features in patients with polymyositis, dermatomyositis, and immune mediated necrotizing myopathy. In this published study, we showed that each type of myositis has a characteristic pattern of muscle involvement on MRI. For example, dermatomyositis patients are distinguished by prevalent fascial edema. We also showed that fatty replacement of muscle occurs early during the course of disease. Moreover, we found that anti-SRP positive patients have more severe MRI findings than anti-HMGCR positive patients. In collaboration with Dr. Lisa Rider, Dr. Fred Miller, and their colleagues at NIEHS, we have sought to determine whether pediatric myositis patients ever have anti-HMGCR autoantibodies. We screened a large cohort of pediatric myositis cases and found that 1% were anti-HMGCR positive. Interestingly, these patients have a unique immunogenetic risk factor not found in adult patients with this form of myositis. This work has been submitted for publication.